Rear screen projection television displays have found great consumer interest due to a large image size. A primary disadvantage is the set volume and depth. There is great desire to reduce set size but this is practically difficult and or costly with present CRT based projection systems.
Micro-display based systems such as LCD, DMD, or LCOS can achieve much smaller system volume than for CRT projection displays. This is due to a smaller engine size and projection from a single projection lens as opposed to three for CRT systems. Very attractive system dimensions have been announced for these systems. Recent product announcements include a 55-inch diagonal wide screen set with a system depth of 18 inches, and a 50-inch diagonal wide screen set with a depth of 35 cm. In these systems, narrow depth is obtained by using a wide-angle projection lens. However, there is a physical limit as to how narrow wide screen systems can be using this method, before the projection system begins to obscure the light path.
Other approaches to reduced system depth include using a polarized folded path (U.S. Pat. No. 573,324), and use of an off-axis aspherical mirror. Both approaches have difficulties that result in reduced picture performance and increased system cost. Furthermore, the height of the sets must be significantly greater to use these concepts.
It is advantageous to reduce system depth beyond what is possible with present art on-axis projection systems. Projection systems become dramatically more attractive as the system depth and height are reduced. New micro-display technologies, such as LCD, DMD, and LCOS, are able to reduce system volume significantly and this should help drive consumer acceptance. As compact dimensions are achieved with present approaches there will be great pressure to reduce system volume even more.
By introducing optical offset into the projection path the set depth can be reduced while maintaining a modest pedestal height. By “optical offset” of a lens or lens system from an object, is meant that the optical axis of the lens or lens system passes a distance (the offset) away from the center of the object. The term “optical axis” of a lens or lens system is well understood in the art; its direction may change depending on reflective or refractive elements placed in its path. For example, a projection lens is optically offset from an object whose image it is projecting, if the projection lens' optical axis (in either direction) does not pass through the center of the object.
Shifting the projection lens vertically creates an optical offset. The image then strikes the viewing screen with an offset angle. To compensate for this angle the Fresnel lens of the viewing screen is correspondingly offset so that the image exits the screen normally.
Rear projection systems typically use short focal length projection lenses, which are placed on-axis. The short focal length results in a short throw distance from lens to screen, which leads to compact system dimensions. Further, the lens is placed on-axis. That is, the rotational axis of symmetry for the projection lens intercepts the image device at the center of the picture. The viewing screen typically consists of a Fresnel lens to largely collimate the light, and a light spreading layer for distributing the light in the viewing space. Light from the center of the image strikes the screen at normal incidence. Thus the optical axis of the Fresnel lens is substantially coincident with the optical axis of the projection lens.
Accordingly, in one aspect of the invention, an offset rear projection display system for displaying an image of an object as a projected image includes a projection lens optically offset from the center of the object, a screen including a light spreading layer, and a mirror disposed to reflect the projected image of the object projected by the projection lens onto the screen.
In another aspect of the invention, a rear projection method that displays an image display of an object onto a screen includes arranging a projection lens to be optically offset from the object, projecting an image of the object from the optically offset projection lens onto a mirror, and reflecting the image off the mirror and onto the screen.
In yet another aspect of the invention, a display system for projecting an image of an object includes a screen and light projection means for projecting the image of the object onto the screen. The light projection means includes a projection lens optically offset from the center of the object.